It is early in the calibration run of the Large Hadron Collider. Physicists monitoring the Transition Radiation Tracker (TRT) of the inner detector of ATLAS have found some unusual lepton tracks which seem to come the decay of from particles close to the beamline.


Here are some things you need to understand to analyze this data:

• Each event has four leptons (muons or electrons)
• Muons are indicated by a hit in the muon chamber
• Virtual Z bosons are thought to be involved as intermediate particles; individual Z bosons can decay into electrons, muons. or taus (but not not a mixture)
• To understand these unknown particles close to the beamline, we need to create a mass plot – a histogram to show their most likely mass, assuming they are the same particle
• Momentum is conserved
• Energy (relativistic) is conserved using E²=p²+M².

Physicists have made cuts to the already small data set which has eliminated all but events with muons and/or electrons. Tracking in the TRT, along with the known strong magnetic field, yields enough information to give the momentum of each lepton. The data you will see gives momentum of each lepton in GeV. A muon hit is indicated by a 1 on the Muon Ch(amber) column for each lepton. A 0 indicates that no muon hit was recorded and that the lepton is therefore most likely to be an electron.


There is a bit of excitement in the TRT community because theory suggests that a Higgs particle – predicted by theorists and sought by experimentalists – can decay into two virtual Z bosons, which may then decay into leptons as described above. Can this be the Higgs? The mass plot may help us to find out and, if it is what we seek, give the first indication of the mass of this new particle.


The data is in an Excel Spreadsheet here: trtsig1.xls